High performance disk drives use information magnetically written on a disk to determine the position of the read/write heads. There are two methods in common use. One employs an entire disk surface containing exclusively position information. This surface is dedicated to servo information and all heads confronting other surfaces and carried by the same actuator are assumed to follow when the servo surface head is moved from track location to track location. The other of the two methods, sector servo, embeds the servo information on all disk surfaces between the data information. The advantages of dedicated servo are a much higher sample rate and simpler, faster head switching. Both of these features improve drive performance. The major disadvantage is that for files with many disks, shifting occurs between the disks at one end of the stack and disks at the other end of the stack. This effect can be reduced by half by placing the dedicated surface in the center of the stack, which does not solve the problem and further creates other difficulties. In such a structure complex or costly shielding may be required or the surface adjacent the servo surface may not be used for data because of the magnetic coupling that would occur when that surface was being written while the servo is trying to read its information. Another manner used to produce more accurate alignment between the data track and the corresponding data head is to provide a servo reference track on the servo surface and on each data surface. By using the data surface servo information, static and dynamic misregistration between the dedicated and data surfaces can be reduced.
The use of reference tracks to improve track following operations by dynamically modifying the servo control signal with a supplementary PES correction signal is described in U.S. Pat. No. 4,136,365. A misposition error correction servo signal counters a previously measured, cyclic off center transducer alignment error. This signal is obtained by measuring off center track misalignment of each data head at various circumferential positions. Misposition errors at such various circumferential positions are sampled for each head, stored and later recalled and applied to the servo controller during a subsequent read/write operation. This technique holds the supplemental PES correction constant until the next sample time arrives. Although this is simple and involves the least amount of computation, it is inaccurate since the actual PES is changing between samples with the result that limited performance enhancement is obtained.
The purpose of the reference track is to correct for radial mechanical motion. However, there is usually tangential motion as well. Most servo patterns must fall within a narrow timing window in order to be demodulated into a position error signal (PES). If the tangential, mechanical motion is large enough, the servo signal written on the data surface may move out of the timing window and fail to demodulate into a PES signal.